The present invention relates to fuel cell power generating systems, and to methods of providing electrical power to a load, or to loads at different voltages from a fuel cell power system.
Fuel cells are well known in the art. A fuel cell is an electrochemical device which reacts a fuel and an oxidant to produce electricity and water. A typical fuel supplied to a fuel cell is hydrogen, and a typical oxidant supplied to a fuel cell is oxygen (or ambient air). Other fuels or oxidants can be employed depending upon the operational conditions.
The basic process in a fuel cell is highly efficient, and for those fuel cells fueled directly by hydrogen, pollution free. Further, since fuel cells can be assembled into stacks of various sizes, power systems have been developed to produce a wide range of electrical power outputs and thus can be employed in numerous commercial applications. The teachings of prior art patents, U.S. Pat. Nos. 4,599,282; 4,590,135; 4,689,280; 5,242,764; 5,858,569; 5,981,098; 6,013,386; 6,017,648; 6,030,718; 6,040,072; 6,040,076; 6,096,449; 6,132,895; 6,171,720; 6,207,308; 6,218,039; 6,261,710 are incorporated by reference herein.
In a fuel cell, hydrogen gas is introduced at a first electrode (anode) where it reacts electrochemically in the presence of a catalyst to produce electrons and protons. The electrons are circulated from the first electrode to a second electrode (cathode) through an electrical circuit which couples these respective electrodes. Further, the protons pass through an electrolyte to the second electrode (cathode). Simultaneously, an oxidant, such as oxygen gas, (or air), is introduced to the second electrode where the oxidant reacts electrochemically in the presence of the catalyst and is combined with the electrons from the electrical circuit and the protons (having come across the electrolyte) thus forming water. This reaction further completes the electrical circuit.
The following half cell reactions take place:
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
(xc2xd)O2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
As noted above, the fuel-side electrode is the anode, and the oxygen-side electrode is the cathode. The external electric circuit conveys the generated electrical current and can thus extract electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate half cell reactions occurring in the fuel cell less its internal losses.
Experience has shown that a single fuel cell membrane electrode assembly of a typical design produces a useful voltage of only about 0.45 to about 0.7 volts D.C. under a load. In view of this, practical fuel cell power plants have been assembled from multiple cells stacked together such that they are electrically connected in series. Prior art fuel cells are typically configured as stacks, and have electrodes in the form of conductive plates. The conductive plates come into contact with one another so the voltages of the fuel cells electrically add in series. As would be expected, the more portions that are added to the stack, the greater the output voltage.
For example, U.S. Pat. No. 5,972,530 to Shelekhin et al. (incorporated herein by reference) describes a fuel cell stack configuration including bipolar fluid flow or separator plates. Each plate includes plate cooling channels and air distribution holes along edges of the assemblies. The bipolar fluid flow plates have a cathode flow field on one major surface (the cathode side), and an anode flow field on the opposite major surface (the anode side). The bipolar fluid flow plates are made of a material that is sufficiently strong to withstand fuel cell operating conditions, that is electrically conductive, and that is chemically inert, such as graphite, titanium, niobium, titanium oxide, stainless steel, carbon composites, or electroplated materials. Membrane electrode assemblies (MEAs) are sandwiched between respective pairs of bipolar fluid flow plates. Each membrane electrode assembly includes a polymer electrolyte membrane (PEM) and electrode material on each side of the PEM. The electrode material on one side of the polymer electrolyte membrane defines an anode and the electrode material on the other side of the polymer electrolyte membrane defines a cathode. The anode is in contact with the anode side of one fuel flow plate in the stack and the cathode is in contact with the cathode side of another fuel flow plate in the stack. While U.S. Pat. No. 5,972,530 describes an air-cooled arrangement, U.S. Pat. No. 5,230,966 (incorporated herein by reference) discloses a liquid cooled arrangement.
In this stack configuration, only a single output voltage is available, while multiple voltages may be desired. It is not convenient to tap voltages from the stack instead of providing a single voltage from the stack as a whole. Traditionally, high voltage outputs from stacks have been desired because power conversion circuitry can be better used with higher voltages. If lower voltages are desired, power conversion circuitry is typically used to convert the output of the stack to the desired voltage. Different customers or users of a fuel cell system may require multiple supplies of smaller voltages than the combined voltage produced a fuel cell stack. The invention described below addresses this issue.